Santana Formation

Description

Brazil’s Santana Formation is one of those Early Cretaceous rock formations which preserve an environment that was literally fit to bursting with pterosaur genera.
It is located in an area known as the Araripe Plateau. At this time, the Albian Stage of the Early Cretaceous, South America was still part of the massive southern supercontinent of Gondwana.

The Setting

At this time much of the continent was also underwater, flooded by parts of the young Atlantic Ocean.
The formation shows an ecosystem of lagoons and coastal flats, the right kind of environment to preserve light-boned pterosaurs and the other animals of their environment in impressive detail. Conifers are some of the plants found in the Santana Formation.
Much of this site has been under siege due to illegal mining, and there have been efforts to protect any particularly fossiliferous areas of the formation.
The bottom of the food web was occupied by a variety of fish in many different ecological niches and at many size classes.

Fish

Among the smallest were the plankton-eaters like the highly speciose, long-lasting genus Leptolepis. This was a foot-long animal, one of the earliest bony fish to actually have a large amount of bone in its body. It evolved during the Jurassic Period and went extinct in the Early Cretaceous.
It was also one of the earliest teleosts, the fish with a stretchy jaw, just as many other well-known fish today have.
Another well-known fish from this formation is the small Tharrias.
It was a small, pelagic swimmer like Leptolepis and was probably a schooling fish that preyed on smaller vertebrates, as well as ostracods. These are tiny crustaceans that swam through the water at almost all levels, serving as prey for pelagic bony fish.
Other fish from the formation include detritivores and benthic species like rays and skates. There are plenty of predators in this formation too, and at various levels of the Santana lagoons.
One of these was the ichthyodectid Cladocyclus. It looked like a fanged tarpon but with a much longer body by comparison, although it was just between one and two meters long. It was a speedy pursuit predator that snapped up small fish in its jaws.
Cladocyclus species were very widespread across Gondwana in the Early Cretaceous, existing in a variety of coastal biomes. Fossils have also been found in the Kem Kem Beds of Morocco.
Another very widespread fish genus was Lepidotes. The very largest of these were also similar in size to the fast Cladocyclus but were stocky and durophagous, preying on shellfish with their crushing teeth.
They also existed for an abnormally long time, starting out in the Early Jurassic and ending up going extinct during the Early Cretaceous.
These fish had a number of predators of course and the largest predators here were non-avialan dinosaurs.

Theropods

The very biggest of these here was Irritator. It was medium-sized in comparison to its relative Spinosaurus but was still quite a sizable hunter capable of taking on big fish. It was also a spinosaur, and probably also had similar features.
The Spinosauridae comprises two subfamilies, the Spinosaurinae and Baryonychinae. The members of the latter subfamily are known from nearly complete remains, which include the legs so there it no doubt that these were mainly wading predators. Spinosaurus is the only member of the former that is known from very good remains, and these show it to be a low-slung, short-legged aquatic specialist.
Irritator itself is only known from its skull, and not a good skull at that. Instead, it was heavily doctored at the time of discovery and paleontologist David Martill had problems uncovering the actual bone. The name derives from the feeling of irritation he and his colleagues felt while trying to find the actual bones under the reconstructed remains.
Other than this, we know that Irritator was probably somewhere between 7 and 8 meters long and if it was anything like Spinosaurus itself, it was probably very low-slung too.
We also know that it was not just a fisher, but probably a generalist that took carrion when it had the opportunity. One particularly tantalizing fossil shows an Irritator tooth lodged in the leg bone of a pterosaur.
There is also a small tyrannosauroid from this formation, an animal known as Santanaraptor.
It was a cursorial predator of about 1.25 meters, hardly anything when considering the huge Irritator. However, until recently tyrannosauroids were not even considered as being part of the Gondwanan dinosaur fauna.
In reality, there was a whole clade of them, the Megaraptora, which were dominant predators in later stages of the Cretaceous. The third theropod known from good remains in the Santana is Mirischia.
Mirischia is quite a well-known and smallish dinosaur, around 2.1 meters long and 10 kilograms in weight. It might be either a compsognathid or a basal tyrannosauroid, according to paleontologist Darren Naish.

Pterosaurs

The pterosaur diversity however, was much more stunning, with a number of fishing pterosaurs coexisting peacefully alongside tapejarids, the possible large herbivores of the Santana Formation.
There are no herbivores among the non-avian dinosaurs so they were clearly feeding on the fish, on one another and the pterosaurs.
This is yet another Gondwanan coastal ecosystem turned completely on its head.
Two pterosaurs that lasted for just the Albian Stage of the Cretaceous and only in Santana were thalassodromids. These were Thalassodromeus and Tupuxuara. Both of these were azhdarchoids and very capable on land. They had massive crests, and longer beaks and heads than the tapejarids, which they somewhat resembled.
However, thy were closer related to the azhdarchids proper, and were more predatory than the Tapejaridae.
The genus Tupuxuara contains three species, and all have different crest profiles. The largest one is T. leonardii, with a rounded end to its crest. This genus probably had a wingspan somewhere over 5.5 meters across, hardly shabby for any kind of pterosaur.
There must have been effective niche partitioning between these pterosaurs or coexistence would be impossible. Thalassodromeus in particular would have been much more predaceous than its relative. Its massive head contained a pointed, almost azhdarchid-like beak and a huge somewhat back-swept crest that took up a large percentage of its skull. It was probably more dependent on terrestrial stalking than its relatives.
Tapejara was one of the smaller pterosaurs here, with a 3.5-meter wingspan. It had a small, short head, and the type species T. wellnhoferi had a small, rounded crest and it probably was the main herbivore of the Santana Formation.
It might have behaved like a parrot, crushing fruits in its beak and probably also having a hand in spreading the seeds of newly-evolving angiosperm plants. It may have also taken small animals, anything that could fit inside its jaws.
The biggest pterosaur here was the massive ornithocheirid Tropeognathus. It starred in an episode of the BBC miniseries, Walking with Dinosaurs. The migratory journey of the animal across the Atlantic Ocean felt almost like a Shakespearean tragedy in the Mesozoic, with the Tropeognathus finally dying due to his great age.
While poignant and emotional, the episode was full of holes and inaccuracies. For one, the taxonomy of many of the featured animals was all over the place.
The main herbivore in the episode was Iguanodon, shown living in both Europe and North America. Unfortunately for the genus, it has recently been under a taxonomic cull, and the North American species has been reclassified as Dakotadon. Also the armored dinosaur Polacanthus did not exist in North America, though its relative Gastonia did.
The big dromaeosaur Utahraptor did not live anywhere close to Europe, as its name quite obviously suggests. There are, however, unnamed fragmentary bones of dromaeosaurs from England at the time.
Also, Tropeognathus is identified as Ornithocheirus. This might be due to the genus being a rather casual dumping ground for various other pterosaur genera. The name was used to describe bones from England’s Cambridge Greensand in 1869 by Harry Govier Seeley himself.
The holotype was unlucky enough to be lumped into Pterodactylus as usual, but Seeley identified differences between the two taxa. Ultimately in 1874, Richard Owen split the genus apart, creating the name Coloborhynchus, which is now full of multiple species.
The Brazilian genus Tropeognathus was named by Peter Wellnhofer in 1987, but he kept reclassifying it, particularly as a species of Ornithocheirus. This turned Tropeognathus mesembrinus into Ornithocheirus mesembrinus. The documentary production team probably used this classification when deciding upon their protagonist.
Also, it was depicted as a huge, 12-meter colossus, as based on some then-undescribed large bones from the Santana Formation. Currently the biggest individuals are estimated to have an 8.2-meter wingspan. Not as titanic, but certainly not small either.
These really were the largest pterosaurs of the Early Cretaceous. Later on, they would be outclassed by the big azhdarchids with their 10-meter spans and giraffe-like height. Tropeognathus probably stood as high as a man when on all fours.
It was an expert fisher with long, powerful albatross-like wings. The beak bore two crests, one on the lower jaw and one on the upper. These were probably used for display as the documentary showed us, although the behavior is still regarded as speculative.

Crocodylomorphs

The stem-crocodile Araripesuchus is another element of the Santana fauna. These animals, nicknamed 'DogCroc' were omnivorous and fast-moving land-dwellers with soft noses and mammal-like teeth. They were widespread around Gondwana and lasted a remarkably long time.
The type species, A. gomesii, hails from the Santana Formation. They lived right until the very end of the dinosaur age, about 66 million years ago in Madagascar. Measuring a meter to about 1.8 meters long, Araripesuchus would have been a scavenger, while also rooting for tubers underground.